1. Field of the Invention
This invention relates to solar heat collecting apparatus, and, more particularly, to solar heat collecting apparatus in which a backup energy source, such as electricity, is used to heat a fluid under conditions of insufficient solar heating, and also in which an opacity of a translucent structure is controlled to prevent overheating of a fluid within the apparatus.
2. Summary of the Background Information
Solar heat collectors generally include translucent walls enclosing tubing in which a fluid, such as water, is circulated to absorb heat from direct sunlight striking the tubing and from the air held within the transparent walls, which is also heated by solar radiation. The solar heat collector may additionally include thermally conductive structures, such as metal fins and plates, which are arranged to absorb solar radiation and to transmit heat to the fluid. The transparent walls may be arranged as a dome, facing upward, and may further be arranged as a dome within a dome. A dome-shaped structure may be supported by a tubular frame through which a fluid flows to be heated.
One problem with conventional solar heat collectors arises from the fact that such systems are often used to heat water for household use in climates that are subject to freezing conditions. When freezing conditions last long enough, particularly at night while heating from solar radiation cannot occur, water within the solar heat collector may freeze, blocking the circulation within the tubing and possibly causing serious damage, such as bursting the tubing due to the expansion. To avoid such an occurrence, antifreeze is mixed with the water being circulated within the solar heat collector, lowering the freezing temperature of the resulting mixture to a level at which the lowest temperature expected to occur within the solar heat collector. Since the addition of an antifreeze to the water makes the water unsuitable for domestic use, the water heated within the conventional solar heat collector can only be used as an inlet to a heat exchanger, in which potable water is in turn heated by the solution of antifreeze in water circulating through the solar heat collector.
Another problem with conventional solar heat collectors arises from the fact that, under sunny and warm climatic conditions, particularly when there is little need for the heated fluid produced within the heat collector, temperatures within the solar heat collector may rise to a point at which damage is done, particularly if the fluid within the heat collector is brought to a boil.